In the flat panel display, such as the liquid crystal display (LCD), the Organic Light-Emitting Display or Inorganic Light-Emitting Display, the thin film transistor is generally employed to be a switch element to control the operation of the pixel or employed to be a drive element to drive the pixel. The thin film transistor generally can be categorized into two kinds, amorphous silicon (a-Si) and polysilicon (Poly-Si) according to the silicon thin film property.
Because the defect issue of the amorphous silicon itself, such as too many defects result in low on state current, low mobility, poor stability, which is restricted in application. For compensating the defect of the amorphous silicon itself and expanding the application field, The LTPS (Low Temperature Poly-Silicon) technology is born at the right moment.
Because of the atom alignment rule, the carrier mobility of the LTPS is high (10-300 cm2/Vs). When it is applied in electronic elements of the Thin Film Transistor (TFT), it can make the TFT possess higher driving current. Therefore, in the manufacture process of the TFT, the LTPS thin film is widely utilized to be material of the active layer, which is one of the core structures of the TFT.
The liquid crystal display utilizing LTPS TFT possesses advantages of high resolution, fast response, high brightness, high aperture ratio, et cetera. Because the silicon crystallization of the LTPS has better order than the amorphous silicon, it makes the electron mobility higher relatively above 100 times, and in the meantime, the peripheral driving circuit is manufactured on the glass substrate to achieve the objective of the system integration to save the space and the cost of the driving IC; meanwhile, cause of the driving IC circuit directly manufactured on the panel, the external connection points of the assembly can be can be diminished to raise the reliability. The maintenance is easier and the assembly process time can be shortened. Thus, the time schedule of the application system design is reduced and the design possibility is enlarged.
However, with the constant decrease of the size of the thin film transistor element, the obvious short channel effect appears to the LTPS TFT, which is mainly caused by the strong electrical field at the drain end. The effects, such as hot carrier effect caused thereby will influence the reliability and stability of the element. At present, a common utilized method is to introduce the Lightly Doped Drain (LDD) or the Offset structure. However, manufacture LDD structure requires one additional photolithography process, and the offset can easily occur in the alignment; the Offset structure might have chance to decrease the switch on current of the element.